Modern cable-television systems tend to use the return path (i.e., subscriber unit to head end communication) inefficiently. Specifically, since subscriber units (such as set-top boxes and two-way cable ready television sets) transmit messages infrequently, the aggregate channel is lightly utilized. In typical existing systems, there is no opportunity to take advantage of unutilized capacity for other purposes, i.e. by taking advantage of statistical gain. However, new architectures are emerging that incorporate packet switched networks (such as Internet Protocol, Asynchronous Transfer Mode or Ethernet based networks) into the access network for the general purpose of carrying packet-based services, such as Internet access, telephony, and network management. It is highly desirable to also carry subscriber unit return traffic over the same packet switched network, thereby making more effective use of expensive facilities such as optical fibers.
It should be noted that these packet switched networks often do not transport a common timing reference, as do circuit transmission systems such as SONET. Thus, systems attached to them do not have a common time base or frequency reference.
Prior art techniques exist for efficiently transporting return signals for cable television systems over a packet network. For example, U.S. Pat. Ser. No. 10/162,461, entitled SYSTEM FOR RETURN BANDWIDTH MINIMIZATION USING SELECTIVE SAMPLING, by Smith, et al., describes a system for efficiently transporting return signals for cable television systems over a packet switched network. Smith et al., teaches a system that comprises a plurality of subscriber units, a plurality of nodes, a packet network, and a head end terminal. Subscriber units occasionally send a message in the form of a burst of modulated signal over a channel to a node. In a node, a selective sampler samples an upstream channel and discriminates valid signal from noise and idle channel conditions in the sample stream. A packetizer and packet forwarder packetizes and forwards samples representing a valid signal burst over the packet switched network to the head end terminal. The head end terminal depacketizes samples and converts them back to an analog broadband signal, which is transmitted over a coaxial cable to a demodulator. The demodulator processes signal bursts from the channel, and forwards the resulting messages into the interactive television system's head end infrastructure.
The above system does provide for a system for communicating over a packet switched network, when a polled or Aloha media access control (MAC) layer, such as that defined in American National Standard ANSI/SCTE 55-1 (2002), is used. If instead a time-division multiple access (TDMA) MAC, such as that defined in American National Standard ANSI/SCTE 55-2 (2002), is utilized, the system becomes highly sensitive to delay jitter. A TDMA system has a frame structure composed of fixed length time slots. A subscriber unit may transmit only at the beginning of a time slot which has been assigned to it by the interactive television system using a Media Access Control protocol. Each subscriber unit must know (e.g., to within 1.5 symbol times) the beginning time of each frame and slot, so that it may transmit only when it is allowed to do so, taking into account differing propagation delays between each subscriber unit and the head end. A ranging offset, which is determined by a feedback control loop, is added to a local clock at each subscriber unit, where the ranging offset is equal to the fixed delay between the STB and the head end. The feedback control loop is designed to compensate for long term variation in delay, such as might result from clock drift or thermal expansion and contraction of outdoor cables. However, it cannot compensate for delay jitter, as occurs due to effects of scheduling and blocking in packet switched networks. Thus, any system that inserts a packet switched network into the return path without compensating for delay jitter will cause instability in a TDMA system, rendering it non-functional. Therefore, a need exists for a method and apparatus for reducing delay jitter within cable television systems employing a TDMA system protocol.